The work function of nickel oxide (NiOx) electrodes was tuned by the covalent attachment of commercially available as well as specially synthesized triethoxysilane functionalized molecules with a range of dipole moments. The presence of the silane molecular layers on the NiOx surface was verified using Fourier Transform Infrared (FTIR) spectroscopy and contact angle measurements. While these tests indicated the surface coverage was incomplete, Kelvin probe measurements showed that the coverage was sufficient to change the work function of the NiOx across a range of ∼900 meV. Density functional theory (DFT) calculations of the dipole moments of the isolated molecules correlated well with the measured work function changes.

This chapter covers the largest set of projects in SERIIUS that can be viewed overall as having three distinct themes. First is the development of new low-capital thin-film photovoltaic (PV) technologies by high-throughput manufacturing processes with a focus on solution-based roll-to-roll processing. This effort was highly integrated with the development of thin glass substrates and packaging from Corning, and focused on absorbers made from CuInGaSe2, CuZnSnSeS, organic molecules such as fluorinated polymers, perovskite solar cells based on methyl ammonium lead iodide (MAPbI), and silicon. Significant progress, including world-record devices, was achieved in a number of areas, and substantial improvements in lifetime were attained. Second is an assessment of the stability and failure mechanisms of PV in hot/dry and hot/wet climates. This work included assessing PV module stability across the seven climate zones of India and in the United States. The importance of dust and particulates was assessed. New failure mechanisms were identified, in particular to the hot/dry and hot/wet climates, and an international working group was convened. Coupled to developing new PV technology and understanding degradation pathways, an effort was made in multiscale modeling – from atoms to modules – to begin to connect the basic science to the ultimate deployability of the PV devices and modules. Here, the potential importance of bifacial modules was assessed.

The morphology of the bulk heterojunction absorber layer in an organic photovoltaic (OPV) device has a profound effect on the electrical properties and efficiency of the device. Previous work has consistently demonstrated that the solubilizing side-chains of the donor material affect these properties and device performance in a non-trivial way. Here, using Time-Resolved Microwave Conductivity (TRMC), we show by direct measurements of carrier lifetimes that the choice of side chains can also make a substantial difference in photocarrier dynamics. We have previously demonstrated a correlation between peak photoconductance measured by TRMC and device efficiencies; here, we demonstrate that TRMC photocarrier dynamics have an important bearing on device performance in a case study of devices made from donor materials with linear vs. branched side-chains and with variable active layer thicknesses. We use Grazing-Incidence Wide Angle X-ray Scattering to elucidate the cause of the different carrier lifetimes as a function of different aggregation behavior in the polymers. Ultimately, the results help establish TRMC as a technique for screening OPV donor materials whose devices maintain performance in thick active layers (>250 nm) designed to improve light harvesting, film reproducibility, and ease of processing. (C) 2017 Published by Elsevier B.V.

We perform molecular dynamics simulations of poly(benzodithiophene-thienopyrrolodione) (BDT-TPD) oligomers in order to evaluate the accuracy with which unoptimized molecular models can predict experimentally characterized morphologies. The predicted morphologies are characterized using simulated grazing-incidence X-ray scattering (GIXS) and compared to the experimental scattering patterns. We find that approximating the aromatic rings in BDT-TPD with rigid bodies, rather than combinations of bond, angle, and dihedral constraints, results in 14% lower computational cost and provides nearly equivalent structural predictions compared to the flexible model case. The predicted glass transition temperature of BDT-TPD (410 ± 32 K) is found to be in agreement with experiments. Predicted morphologies demonstrate short-range structural order due to stacking of the chain backbones (π–π stacking around 3.9 Å), and long-range spatial correlations due to the self-organization of backbone stacks into "ribbons" (lamellar ordering around 20.9 Å), representing the best-to-date computational predictions of structure of complex conjugated oligomers. We find that expensive simulated annealing schedules are not needed to predict experimental structures here, with instantaneous quenches providing nearly equivalent predictions at a fraction of the computational cost of annealing. We therefore suggest utilizing rigid bodies and fast cooling schedules for high-throughput screening studies of semiflexible polymers and oligomers to utilize their significant computational benefits where appropriate.

The Durable Module Materials Consortium, or DuraMat, is a consortium led by the National Renewable Energy Laboratory (NREL), in partnership with Sandia National Laboratories, Lawrence Berkeley National Laboratory, and SLAC National Accelerator Laboratory, that is intended to accelerate the development of module materials for photovoltaics (PV) and lower the cost of electricity generated by solar power. The overarching goal of DuraMat is to discover, develop, de-risk, and enable the rapid commercialization of new materials and designs for photovoltaic (PV) modules that can improve performance and lifetime with the goal of enabling a solar levelized cost of electricity (LCOE) of less than $0.03 per kilowatt hour. As part of the Energy Materials Network (EMN), DuraMat brings together the best of the national lab and university research infrastructure in active collaboration with the PV and supply-chain industries to achieve this goal. DuraMat provides a unique opportunity to employ the Materials Genome Initiative methodology in the EMN framework in order to accelerate advances in materials and module designs and increase US competitiveness.

Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is less selective for holes in an inverted-architecture organic photovoltaic (OPV) than it is in a conventional-architecture OPV device due to differences between the interfacial-PSS concentration at the top and bottom of the PEDOT:PSS layer. In this work, thin layers of polysulfonic acids are inserted between the poly(3-hexylthiophene) (P3HT):indene-C 60 bisadduct (ICBA) bulk heterojunction (BHJ) active layer and PEDOT:PSS to create a higher concentration of acid at this interface and, therefore, mimic the distribution of materials present in a conventional device. Upon thermal annealing, this acid layer oxidizes P3HT, creating a thin p -type interlayer of P3HT + /acid − on top of the BHJ. Using X-ray absorption spectroscopy, Kelvin probe, and ellipsometry measurements, this P3HT + /acid − layer is shown to be insoluble in water, indicating it remains intact during the subsequent deposition of PEDOT:PSS. Current density–voltage measurements show this doped interlayer reduces injected dark current while increasing both open-circuit voltage and fill factor through the creation of a more hole selective BHJ-PEDOT:PSS interface.

This work focuses on the role of humidity in the formation of ZnO thin films from a reactive diethylzinc precursor solution for use as the electron contact layer (ECL) in organic photovoltaic (OPV) devices. This method is well suited for flexible devices because the films are annealed at 120 degrees C, making the process compatible with polymer substrates. ZnO films were prepared by spin coating and annealing at different relative humidity (RH) levels. It is found that RH during coating and annealing affects the chemical and physical properties of the ZnO films. Using x-ray photoelectron spectroscopy it is found that increasing RH during the formation steps produces a more stoichiometric oxide and a higher Zn/O ratio. Spectroscopic ellipsometry data shows a small decrease in the optical band gap with increased humidity, consistent with a more stoichiometric oxide. Kelvin probe measurements show that increased RH during formation results in a larger work function (i.e. further from vacuum). Consistent with these data, but counter to what might be expected, when these ZnO films are used as ECLs in OPV devices those with ZnO ECLs processed in low RH (less stoichiometric) had higher power conversion efficiency than those with high-RH processed ZnO due to improved open-circuit voltage. The increase in open-circuit voltage with decreasing humidity was observed with two different donor polymers and fullerene acceptors, which shows the trend is due to changes in ZnO. The observed changes in open-circuit voltage follow the same trend as the ZnO work function indicating that the increase in open-circuit voltage with decreasing humidity is the result of improved energetics at the interface between the bulk-heterojunction and the ZnO layer due to a vacuum level shift. (c) 2016 Elsevier B.V. All rights reserved.

Currently the exact recombination mechanism of free carriers in organic photovoltaic (OPV) devices is poorly understood. Often a reduced Langevin model is used to describe the decay behavior of electrons and holes. Here we propose a novel, simple kinetic model that accurately describes the decay behavior of free carriers in the PTB7:PCBM organic photovoltaic blend. This model needs to only take into account free and trapped holes in the polymer, and free electrons in the fullerene, to accurately describe the recombination behavior of free carriers as measured by time-resolved microwave conductivity (TRMC). The model is consistent for different PTB7:PCBM blend ratios and spans a light intensity range of over 3 orders of magnitude. The model demonstrates that dark carriers exist in the polymer and interact with photoinduced charge carriers, and that the trapping and detrapping rates of the holes are of high importance to the overall carrier lifetime.

This work investigates the influence of fluorination of both donor and acceptor materials on the generation of free charge carriers in small molecule donor/fullerene acceptor BHJ OPV active layers. A fluorinated and non-fluorinated small molecule analogue were synthesized and their optoelectronic properties characterized. The intrinsic photoconductance of blends of these small molecule donors was investigated using time-resolved microwave conductivity. Blends of the two donor molecules with a traditional non-fluorinated fullerene (PC 70 BM) as well as a fluorinated fullerene (C 60 (CF 3 )2-1) were investigated using 5% and 50% fullerene loading. We demonstrate for the first time that photoconductance in a 50:50 donor:acceptor BHJ blend using a fluorinated fullerene can actually be improved relative to a traditional non-fluorinated fullerene by fluorinating the donor molecule as well.

Solvent additives such as diiodooctane (DIO) are becoming ubiquitous in processing high performance organic photovoltaic (OPV) active layers. Here, we investigate the effects of DIO on the long-term stability of the active layer by studying the photodegradation under ambient white light illumination of the polymer PTB7-Th in pure polymer thin films and in blend films with PC71BM. Using X-ray fluorescence, we directly detect iodine in the active layer films, indicating the presence of residual DIO after casting from solution. Additionally, we show that this residual DIO dramatically decreases the photostability of the active layer. Structural changes in the films upon illumination are probed with grazing-incidence wide-angle X-ray scattering (GIWAXS). FTIR spectroscopy is used to monitor chemical changes in the polymer structure during irradiation in the presence of DIO. Furthermore, we demonstrate that film treatment either with high vacuum (10(-8) Torr) for 60 min or with a high-temperature thermal anneal at 175 degrees C for 30 min removes residual DIO from the film and delays photodegradation. Therefore, when processing polymer solar cells with DIO-containing solutions, it is imperative to remove any trace amounts of DIO from deposited films.

The p-type semiconductor GaInP2 has a nearly ideal bandgap (∼1.83 eV) for hydrogen fuel generation by photoelectrochemical water splitting but is unable to drive this reaction because of misalignment of the semiconductor band edges with the water redox half reactions. Here, we show that attachment of an appropriate conjugated phosphonic acid to the GaInP2 electrode surface improves the band edge alignment, closer to the desired overlap with the water redox potentials. We demonstrate that this surface modification approach is able to adjust the energetic position of the band edges by as much as 0.8 eV, showing that it may be possible to engineer the energetics at the semiconductor/electrolyte interface to allow for unbiased water splitting with a single photoelectrode having a bandgap of less than 2 eV.

Calculations predict energy levels of polymers for OPV, and time-resolved microwave conductivity pre-screens for charge generation ability prior to device fabrication.

The role of the contacts in thin-film, blended heterojunctions (<100 nm thick) organic photovoltaics is explored, specifically considering concepts of carrier selectivity, injection, and extraction efficiency, relative to recombination. Contact effects are investigated by comparing two hole-collecting interlayers: a phosphonic acid monolayer on indium tin oxide (ITO) and a nickel oxide thin film. The interlayers have equivalent work functions (≈5.4 eV) but widely variant energy band offsets relative to the lowest unoccupied molecular orbital of the acceptor (electron blocking versus not), which are coupled to large differences in carrier density. Trends in open-circuit voltages (VOC) as a function of light intensity and temperature are compared and it is concluded that the dominant mechanism limiting VOC for high density of states contacts is free carrier injection, not surface recombination or extraction barriers. Transient photocurrent decay measurements confirm excess reinjected carriers decrease the extraction efficiency via increased recombination and decrease free carrier lifetime, even at high internal electric fields, due to space charge accumulation. These results demonstrate that the energetics and injection dynamics of the interface between interlayers and high carrier density electrodes (typically ITO and metals) must be considered with fabrication and processing of interlayers, in addition to possible carrier selectivity and the interface with the active layer.

In this Perspective, we provide a brief background on the use of aromatic phosphonic acid modifiers for tuning work functions of transparent conducting oxides, for example, zinc oxide (ZnO) and indium tin oxide (ITO). We then introduce our preliminary results in this area using conjugated phosphonic acid molecules, having a substantially larger range of dipole moments than their unconjugated analogues, leading to the tuning of ZnO and ITO electrodes over a 2 eV range as derived from Kelvin probe measurements. We have found that these work function changes are directly correlated to the magnitude and the direction of the computationally derived molecular dipole of the conjugated phosphonic acids, leading to the predictive power of computation to drive the synthesis of new and improved phosphonic acid ligands.